The present invention relates to a semiconductor device having a protrusive electrode and manufacturing method of the same, and more particularly it relates to a semiconductor device with a protrusive electrode to connect the device and other devices electrically, and manufacturing method thereof.
Today, for an electrode pad made of Al (aluminum) or mainly made of Al (will be referred to as electrode pad) on semiconductor substrates, electroless plating is used as one of the methods to form a protrusive electrode that electrically connects the electrode pad and other devices.
The electroless plating can omit steps such as:
a sputtering step that is required to form a barrier metal layer and an electrode in a step of plating;
a photo step that is needed for a pattern formation of a protrusive electrode; and
an etching step that eliminates a resist that is used in the step of pattern formation and a barrier metal that is used in the plating step.
Comparing with an electrolytic plating, the electroless plating can form the electrode pad with fewer steps. So the electroless plating has attracted attention as a method that enables to reduce costs as well as shorten delivery times.
The electroless plating is a method to selectively form a protrusive electrode made of Ni (nickel) or a Ni alloy (will be referred to as protrusive electrode) on the electrode pad. In this process, if an oxide film exists on the surface of the electrode pad, it deeply influences the form and reliability of the protrusive electrode, since the protrusive electrode cannot be formed uniformly on the electrode pad.
Therefore, before doing the electroless plating, the oxide film on the surface of the electrode pad is removed by sodium hydroxide, phosphoric acid, etc., to form a protrusive electrode in good and desired shape. However, a microscopic gap is formed between the protrusive electrode and the protective coat, because the protrusive electrode is developed from the surface of the electrode pad and not chemically joined with the protective coat on the semiconductor substrate.
Referring to FIGS. 5(a) through 5(d), the following description will discuss one example of methods to form a protrusive electrode on a electrode pad by the electroless plating.
FIGS. 5(a) through 5(d) are sectional process drawings of a method to form a protrusive electrode on the electrode pad made of Al or mainly made of Al through a commonly used electroless plating.
FIG. 5(a); On a surface of an electrode pad 22 made of Al or mainly made of Al which is formed on a semiconductor substrate 21, there is an area that is not covered with a protective coat 24. The figure is a section view which shows a step to remove an oxide film 23 that is formed on the uncovered area described above. In this step, the oxide film 23 is completely removed using sodium hydroxide, phosphoric acid, etc. By the way, a formation of an insulating film on the surface of the semiconductor substrate 21 is omitted from the figure.
The oxide film 23 will be formed on the electrode pad 22 again, if the electrode pad 22 from which the oxide film 23 is removed is left as it is. Thus, as the FIG. 5(b) shows, a Zn film 25 is formed on the electrode pad 22 to prevent the re-formation of the oxide film 23. A process to form the Zn film 25 uniformly on the surface of the electrode pad 22 (zincate process) is a preliminary step to form a protrusive electrode 26 (see FIG. 5(c)) by the electroless plating that precipitates Ni or a Ni alloy (will be referred to as Ni). The zincate process is carried out as follows; the electrode pad 22 is dipped into an alkaline solution that contains Zn, and displacement reaction is occurred between Al in the electrode pad 22 and Zn ions in the solution.
FIG. 5(c) is a sectional view that shows a step to form the protrusive electrode 26 through the electroless plating that precipitates Ni. The plating by Ni through the use of the electroless plating is done as the electrode pad 22 on which the Zn film 25 is evenly formed is dipped into electroless Ni plating liquid. On account of this, Zn in the Zn film 25 dissolves in the electroless Ni plating liquid and the displacement reaction between Zn and Ni ions in the electroless Ni plating liquid occurs, then Ni is precipitated on the electrode pad 22. Once Ni that becomes a nucleus is precipitated on the electrode pad 22, due to an autocatalytic reaction (self-reduction reaction) of the electroless Ni plating liquid, Ni is self-precipitated on Ni, and the protrusive electrode 26 is formed.
Since the Zn film 25 is evenly formed in the zincate process as above, Ni uniformly grow on the electrode pad 22 and the protrusive electrode 26 which is well-shaped and small grain size is obtained. However, since the protective coat 24 and the protrusive electrode 26 are not chemically joined, a microscopic gap 27 is formed between these two.
FIG. 5(d) is a sectional view that shows a step to form an immersion Au film 28 on the protrusive electrode 26 by immersion Au plating. The immersion Au plating is done by dipping the electrode pad 22, on which the protrusive electrode 26 is formed, into an immersion Au plating liquid. In this manner, as the figure shows, the immersion Au film 28 is formed on the surface of the protrusive electrode 26. By the way, to prevent corrosion of the electrode pad 22 in a subsequent step of electroless Au plating, the surface of the protrusive electrode 26 must be covered by the immersion Au film 28, by this immersion Au plating.
However, due to extreme narrowness of the gap 27 between the protective coat 24 and the protrusive electrode 26, it is difficult to remove the electroless Ni plating liquid from the gap 27. If the immersion Au plating is done on condition that the electroless Ni plating liquid still remains in the gap 27, the immersion Au plating liquid cannot enter the gap 27 adequately. Thus the immersion Au film 28 that formed in the area where the protective coat 24 faces the surface of the protrusive electrode 26 becomes defective, and leaves areas not plated by the immersion Au film 28 on the surface of the protrusive electrode 26.
In a conventional method of this process, areas not plated by the immersion Au film 28 remain on the surface of the protrusive electrode 26, because the microscopic gap 27 between the protective coat 24 and the protrusive electrode 26 is formed. Therefore, in a subsequent step:
an adhesion between the electrode pad 22 and the protrusive electrode 26 becomes poor, because the electroless Au plating liquid enters the gap 27 and the electrode pad 22 is corroded; and
reliability of the semiconductor device becomes significantly lower, owing to a seepage of the electroless Au plating liquid from the protrusive electrode 26.
So, an object of the present invention is to offer a highly reliable semiconductor device with good adhesion between an electrode pad and a protrusive electrode and the manufacturing method of the device, by completely covering the surface of the protrusive electrode with a defect-free film that is formed on the surface of the protrusive electrode.
By the way, as a method to form an electrode for a semiconductor element, to resolve a problem that a microscopic gap between a protective coat and a metal film (a protrusive electrode) which is formed due to warpage of a wafer, Japanese Laid-Open Patent Application No. 10-125682/1998 (Tokukaihei 10-125682; published on May 15, 1998) discloses:
{circle around (1)} a technique to reduce warpage of the wafer due to the difference of temperature, caused as a plating step and a washing step are done at the same temperature;
{circle around (2)} a technique to reduce warpage of the wafer by attaching a reinforcing frame; and
{circle around (3)} a technique to carry out Ni plating again at room temperature.
However, what the invention disclosed in the application above intends is to reduce the gap between the protective coat and the metal film due to warpage of the wafer. So its object is different from that of the present invention trying to resolve a problem caused by a gap between a protective coat and a protrusive electrode, which is formed because these two are not chemically joined.
Moreover, the technique disclosed in the application above cannot avoid formation of a microscopic gap between the protective coat and the metal film (protrusive electrode), because these two are not chemically joined. Thus the technique disclosed in the application above cannot resolve the problem that the present invention aims to resolve.
An object of the present invention is to provide a highly reliable semiconductor device with good adhesion between an electrode pad and a protrusive electrode and manufacturing method thereof, by covering a surface of the protrusive electrode completely with a defect-free film.
In order to fulfil the above, the method of manufacturing the semiconductor device in accordance with the present invention is a method of manufacturing of a semiconductor device containing a protrusive electrode formed on a part of a surface of an electrode pad on a semiconductor substrate, and a protective coat covering a remaining part of the surface, and including the steps of:
(a) forming the protrusive electrode on the electrode pad;
(b) etching to form a gap between the protrusive electrode and the protective coat after the step (a); and
(c) plating to form a metal film on the surface of the protrusive electrode after the step (b).
In a conventional method of manufacturing a semiconductor device, an Au film is formed as follows:
after removing an oxide film from a surface of a electrode pad made of Al or mainly made of Al, a Zn film is formed by conducting a zincate process (displacement between Al and Zn on the surface) several times;
a protrusive electrode is formed by electroless plating that causes displacement reaction between the Zn film and Ni or a Ni alloy; and
immediately afterward the Au film is formed on the surface by conducting immersion Au plating.
However, since the protective coat and the protrusive electrode on the surface of the electrode pad are not chemically joined, a microscopic gap exists between these two. So, in the step of the immersion Au plating, the immersion Au liquid does not enter the gap adequately, because the gap is extremely narrow. Therefore, it is difficult to cover an area between the surface of the protrusive electrode and the protective coat with the Au film.
Since the surface of the protrusive electrode is not completely covered with the Au film, the electrode pad is corroded in a subsequent step conducting electroless Au plating to the protrusive electrode, and the adhesion between the electrode pad and the protrusive electrode becomes poor. Also, the reliability of the semiconductor device becomes significantly lower, because electroless plating liquid, which is used to form the protrusive electrode in the step of the electroless Au plating, seeps from the electrode.
So, the method of manufacturing of the semiconductor device in accordance with the present invention includes the steps of:
forming the protrusive electrode;
etching to form the gap between the protrusive electrode and the protective coat after the step of forming the electrode; and
plating to form a film on the surface of the protrusive electrode through the plating step as above.
That is to say, the film completely covering the surface of the protrusive electrode, i.e. the defect-free film is formed by conducting the plating step, after the microscopic gap between the protective coat and the protrusive electrode is widened in the etching step to be the one which the plating liquid can enter uniformly. Further, the etching may be done at any temperature. So it may be done at room temperature, for example.
Also, by widening the microscopic gap between the protective coat and the protrusive electrode to be the one which the plating liquid can enter uniformly, the etchant used in the etching step and the plating liquid used in the plating step can be washed out easily. In other words, the cleaning of the etchant and the plating liquid can be done more efficiently.
By the way, although a form and a width of the gap is not fixed as long as the plating liquid can enter the gap uniformly, it is preferable that a distance between the protective coat and the protrusive electrode is kept between 0.05 xcexcm and 1 xcexcm, to form an adequate gap. By keeping the distance as above, the plating liquid can enter the gap easily.
In the arrangement above, in the plating step, the defect-free film covering all over the surface of the protrusive electrode is formed, due to the proper formation of the film in the area between the protrusive electrode and the protective coat. Thus, for instance, when electroless Au plating is conducted to the protrusive electrode, corrosion of the protrusive electrode by electroless Au plating liquid in the area where the protrusive electrode and the electrode pad contact can be prevented, because the film covering all over the surface of the protrusive electrode prevents the entry of the electroless Au plating liquid.
Also, as the film covers every part of the surface, for instance, even if the protrusive electrode is formed by way of electroless plating, electroless plating liquid does not seep from the protrusive electrode. Therefore, it is proved that the reliability of the semiconductor device is increased, since leakage-related failures caused by the seepage of the electroless plating liquid are eradicated.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.